phab.wmfusercontent.org + wmfusercontent.org are both set up to 301->https and send preload-enabled HSTS headers. However, https://wmfusercontent.org still returns a 404 (as it's not configured properly on cache_misc), and the HSTS appspot checker doesn't like 4xx/5xx. If I just added it as a domain pointing at the phabricator applayer, it would 5xx, so that's no better (and not really correct anyways).

Perhaps we can synth a 200 OK for this with a short text/plain output, just to satisfy preload requirements?